The invention relates to a method and apparatus for aligning a portion of textile fabric, and in particular to a method and apparatus with gripping elements moving the fabric across a display surface. At least two independently controllable and movable gripping elements of a display device are brought alongside one another, into contact with, and fastened to, a front edge of a portion of a fabric. The gripping elements are moved substantially parallel to the display surface of the display device until the portion of fabric is laid out with one side flat on the display surface.
From DE 299 08 280 an apparatus for inspecting portions of textile fabric, in particular of towelling, is known. Said apparatus allows initially one side of the portion of fabric to be laid out flat with the aid of movable gripping elements so that the upper side may be inspected. The gripping elements are then moved in the opposite direction, with the result that the portion of fabric is turned over to allow inspection of the second side.
Many production sequences require the portion of fabric to have a specific alignment so that it may, e.g. in a downstream folding installation, be folded in the correct position. Problems with the alignment of the portions of fabric arise particularly when, because of specific production conditions or because of properties of the fabric, the edges of the portion of fabric do not extend at right angles to one another. Thus, for example, installations for manufacturing hand towels are known, in which the edges of the hand towels after sewing extend in the manner of a parallelogram relative to one another. This means that the front edge and the back edge, on the one hand, and the two side edges, on the other hand, extend in each case parallel to one another but the side edges are not at right angles to the front or back edge.
A drawback of the known apparatus is that an alignment of the portions of fabric is not possible.
The object of the present invention is to propose an apparatus for turning a portion of textile fabric, which avoids said drawback.
Said object is achieved according to the invention by a method and an apparatus. The method provides a display surface with an alignment set point. First and second gripping elements are provided which are independently movable across the display surface. A first edge of the fabric is connected to the gripping elements. The gripping elements with the fabric are then moved across the display surface. An offset distance from a second edge of the fabric to the alignment set point is measured and a rate of moving of the gripping elements is adjusted to minimize the offset distance. A control unit is provided which controls the movement of the gripping elements to minimize the offset distance.
According to the invention, the gripping elements are controllable independently of one another and may therefore be positioned independently of one another. This may be realized, for example, by mounting each gripping element on a separate slide, wherein the slides are each driven independently of one another by individually controllable motors. The effect according to the invention is based on the fact that the alignment of the portion of fabric is clearly determined by the shape of an edge of the portion of fabric so long as the portion of fabric is lying flat on the display surface. By means of a feed difference between the gripping elements fastened to the front edge it is therefore possible to vary the alignment of the front edge and hence the alignment of the entire portion of fabric. In said case, however, it should be noted that for alignment of the portion of fabric at the front edge only pulling may be used because pushing forces are not transmissible in a web of fabric. The feed difference between the gripping elements is therefore achieved according to the invention in that the gripping elements are at least temporarily driven at different feed rates. As a result of the rate difference the feed difference is continuously increased, with the result that fold formation in the fabric may be avoided.
The method according to the invention may be implemented particularly easily and the apparatus according to the invention may be of a particularly simple design when, for turning the portion of fabric, two gripping elements are provided, which may be fixed in the corners of the portion of fabric.
It is advantageous when the actual alignment of the portion of fabric, as the latter is laid out on the display surface, may be measured by a position sensor. For said purpose it is possible to use, for example, a distance sensor which is disposed on the display surface in the region of one of the two side edges. From a variation of the distance between the sensor and the side edge it may be inferred that the side edge is not being drawn in parallel to the conveying direction. In said case, for example, the feed rate of one of the gripping elements may be increased by a specific amount until the distance once more corresponds to a specific setpoint distance.
With the method according to the invention, the alignment of the portion of fabric may be adjusted also in the manner of a control loop.
When a portion of fabric is turned over, it is desirable for the portion of fabric to be laid out on the display surface in the correct position, i.e. in accordance with the setpoint alignment, both before and after turning. Where this is required, it is naturally possible to align the portion of fabric in accordance with the proposed method before and after turning in each case with the aid of the position sensor. However, as each aligning operation with the aid of the position sensor takes a specific amount of time, this increases the throughput times and hence the cost of production. With a particularly advantageous process sequence, however, it is possible to eliminate aligning of the portion of fabric after turning.
In said case, first the feed difference between the gripping elements is acquired after the portion of fabric has been laid out in the correct position prior to turning, i.e. with the first side situated underneath. The special implementation of the method for eliminating the aligning operation after turning is based on the discovery that the necessary feed difference for laying out the portion of fabric in the correct position after turning, in terms of its amount, corresponds to the feed difference for aligning the portion of fabric prior to turning. Thus, in order to lay out the portion of fabric in the correct position after turning without remeasuring the alignment, all that is required is to include the necessary feed difference, which was established during the first laying-out operation, in the feed values for moving in the opposite direction to turn the portion of fabric. For complete turning of the portion of fabric, the gripping element with the lowest feed value during the first laying-out operation, in order to turn the portion of fabric, has to be moved in the opposite direction by a feed value which is at least greater than the length of the portion of fabric. The gripping element with the greatest feed value during the first laying-out operation, on the other hand, for turning the portion of fabric is moved in the opposite direction by a feed value, which arises from the sum of the feed value of the first gripping element during the turning operation and twice the acquired feed difference.
When more than two gripping elements are involved in manipulating the portion of fabric, the feed values of the gripping elements between the two outermost gripping elements arise in accordance with the distance between the gripping elements as intermediate values.
Since in the course of turning the portion of fabric a top layer of the fabric is drawn across a bottom layer, if the movement of the gripping elements is not synchronized this may lead to undesirable fold formation in the portion of fabric. It is therefore particularly advantageous when the second gripping element during turning in a first phase is initially moved in the opposite direction by a feed value, which is twice the stored feed difference, while at the same time the first gripping element is initially stationary. Thus, in the first turning phase the double feed difference required between the gripping elements for positionally correct turning is initially compensated. Then, for complete turning of the portion of fabric, in a second phase both gripping elements are synchronously moved in the opposite direction by a feed value, which is at least greater than the length of the portion of fabric.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.